Direct measurement of hairpin-like vortices in the bottom boundary layer of the coastal ocean
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© 2016. American Geophysical Union. All Rights Reserved. Laboratory measurements and numerical modeling at low Reynolds numbers (Reθ<7700) indicate the energy-containing turbulence of boundary layer flows comprises coherent packets of hairpin vortices. Here direct measurements in the bottom boundary layer of the coastal ocean at higher Reynolds numbers (Reθ= 266,150) show tidal flows also contain packets of large vortices separated by periods of more quiescent conditions. The 1452 vortices recorded within a 20 min period are typically aligned along stream (∼8.0° from the mean flow direction) and inclined to the horizontal (∼27.0° from the seabed), with a mean period of occurrence of 4.3 s. These results lend three-dimensional, in situ support to an interpretation of the coastal ocean bottom boundary layer as comprising coherent packets of hairpin vortices. This demonstrates a direct linkage from low Reynolds number experiments to higher Reynolds number flows, permitting fine-scale details of particle transport and pollutant dispersion to be inferred from lower Reynolds number data.
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